The human form of the disease - primary pulmonary hypertension - often leads to cardiovascular complications such as right heart hypertrophy and failure and is frequently lethal, causing around 125-150 deaths per year in the US. It affects up to three people per million population each year.
Presenting their work at the Experimental Biology meeting in San Diego, Dr David Ku and colleagues from the University of Alabama in the US said the findings confirm an earlier study showing that garlic protects against a less lethal form of acute pulmonary hypertension in rats.
The new results however pinpoint the effective ingredient in garlic and demonstrate that it achieves its protective effects through vasorelaxation, or relaxation of the blood vessels.
The researchers tested the benefits of garlic on rats that had induced vasoconstriction of the pulmonary arteries. Half of the animals received a daily supplement of allicin for three weeks, an amount equivalent to two cloves of garlic a day for humans, and another group received garlic with the allicin removed.
Within three weeks, the second group had developed chronic pulmonary hypertension, with markedly increased pulmonary arterial pressure, just as expected. But the allicin group experienced no such increase.
In a separate study also presented at the meeting, Dr Ku's team found that garlic treatment could also protect coronary vascular function and lessen the severity of right heart hypertrophy, two of the serious byproducts of chronic pulmonary hypertension.
Garlic has long been thought to have medicinal properties, with studies suggesting that supplements could help lower blood pressure, reduce cholesterol levels and inhibit platelet function.
Garlic also has been suggested to improve arterial oxygenation associated with pulmonary dysfunction in patients with hepatopulmonary syndrome.
However, researchers meeting under the auspices of the US National Heart, Lung and Blood Institute in 2002 found that there was little scientific data to support many of the claims about garlic.
Dr Ku said this problem is partly due to the difficulty in accurately detecting and quantifying levels of allicin. Using a new analytical method, based on reversed-phased high performance liquid chromatography combined with mass spectrometry with multiple ions reaction monitoring, the team has now demonstrated that varying concentrations of garlic active metabolite from either pure allicin, freshly crushed garlic, or commercial freeze-dried garlic preparations, directly correlate to the extent of their vasorelaxation/vasodepressor responses in isolated pulmonary arteries and in living animals.
Studies are needed to prove similar beneficial effects from the allicin in garlic will occur in humans. However because of its long traditional use in the diet, garlic is known to have low toxicity.